A variety of hormones, neurotransmitters and biologically active substances control, regulate or adjust the functions of living bodies via specific receptors located in cell membranes. Many of these receptors mediate the transmission of intracellular signals by activating guanine nucleotide-binding proteins (hereinafter, sometimes referred to as G proteins) to which the receptor is coupled. Such receptors are generically referred to as G protein coupled receptors.
G protein coupled receptors (hereinafter sometimes termed “GPCR”s) comprise a large superfamily of receptors typically sharing a common structural motif of seven transmembrane helical domains. Some GPCRs do not have seven transmembrane helical domains and instead can be single-spanning transmembrane receptors for cytokines such as erythropoietin, EGF, insulin, insulin-like growth factors I and II, TGF, or potentially multi-polypeptide receptors such as GPIb-V-IX or the collagen receptor that exhibit outside-in-signaling via G proteins. GPCRs play a vital role in the signaling processes that control cellular metabolism, cell growth and motility, inflammation, neuronal signaling, and blood coagulation. G protein coupled receptor proteins also have a very important role as targets for molecules such as hormones, neurotransmitters and physiologically active substances, which molecules control, regulate or adjust the functions of living bodies. For instance, GPCRs include receptors for biogenic amines, e.g., dopamine, epinephrine, histamine, glutamate (metabotropic effect), acetylcholine (muscarinic effect), and serotonin; for lipid mediators of inflammation such as prostaglandins, platelet activating factor, and leukotrienes; for peptide hormones such as calcitonin, C5a anaphylatoxin, follicle stimulating hormone, gonadotropin releasing hormone, neurokinin, oxytocin, and for proteases such as thrombin, trypsin, and factor VIIa/Xa; and for sensory signal mediators, e.g., retinal photopigments and olfactory stimulatory molecules. Each molecule has its own receptor protein which is specific thereto, whereby the specificities of individual physiologically active substances, including specific target cells and organs, specific pharmacological actions, specific action strength, action time, etc., are decided. Thus, GPCRs are a major target for drug action and development.
Although hundreds of G protein coupled receptor genes or cDNAs have been cloned, it is believed that there are still many uncharacterized G protein coupled receptors which have not been recognized as GPCRs, as of yet. GPCRs that lack known agonists are known as orphan receptors. Furthermore, there are currently no effective strategies to directly study the mechanism of receptor-G protein coupling in a controlled fashion under in vivo conditions. Nor is there an understanding of the selective contacts between receptors and G proteins, or the elucidation of the mechanisms of G protein activation by receptors.
Thus, a need remains in the art for compositions useful in defining a strategy that can be used to elucidate and further define selective contact site(s) between receptors and G proteins on the intracellular surface of the cell membrane, as well as a general molecular strategy for use in the facile development and screening of novel therapeutics targeted to receptor-effector interfaces.